D-xylose, a five-carbon sugar, is a naturally occurring carbohydrate present in large quantities in a wide variety of plant materials. It often comprises over 60% of the recoverable sugars derived from hemicelluloses.
Numerous microorganisms such as bacteria and mycelial fungi are known to produce small amounts of ethanol and other products from D-xylose under fermentation conditions. These methods however, do not provide a means for the conversion of D-xylose to ethanol in high yields.
Many yeasts are capable of fermenting hexoses to ethanol anaerobically in high yields. However, no yeasts have been reported to ferment pentoses (or specifically D-xylose) to ethanol effectively, even though many yeasts are capable of both metabolizing pentoses aerobically, and in many cases, producing polyols (e.g., xylitol, and arabitol) as the metabolic by-products. Therefore, D-xylose, which is derived primarily from hemicellulosic materials, has been regarded as a nonfermentable sugar.
Several bacteria (e.g., clostridia) and mycelial fungi (e.g., Mucor spp., Rhizopus spp., Monilia spp., and Fusaria) are known to metabolize as well as ferment D-xylose to ethanol. However, for ethanol production from biomass, yeasts are preferred over bacteria and mycelial fungi. The yeast process for fermenting glucose to ethanol is a relatively simple and well-studied process. On the other hand, bacterial fermentation of D-xylose has not been satisfactory due to the low yield and the undesirable generation of organic acids [e.g., lactic acid] as fermentation products along with ethanol, as confirmed in U.S. Pat. No. 1,857,429. Mycelial fungal fermentation of D-xylose has not been satisfactory either, due to the slow-rate of fermentation and low-yield of ethanol.
The biological conversion of five-carbon sugars derived from hemicellulose to ethanol is important in order to fully utilize biomass so as to produce liquid fuels, especially in view of the fact that vast quantities of hemicellulosic materials are readily available. Unfortunately these materials have been greatly under-utilized due to the lack of the ability of proper organisms to convert D-xylose to ethanol efficiently.
Accordingly, it is the primary object of the present invention to provide a new process for the production of ethanol from D-xylose using novel xylose-fermenting yeast mutants.
It is a further object of the present invention to provide a means for production of ethanol from both D-xylose and other six-carbon sugars, simultaneously. These and other objects will be more apparent from the discussion below.